The invention relates to a headbox or flowbox of a machine for producing a fiber web, in particular a paper web, and particularly relates to a structure and a method of reducing the effective width of a sectional liquid feed to each section of the headbox across its width, in or near the entry region of a turbulence insert in the main stream of stock suspension in the headbox.
A headbox receives pulp stock suspension from a supply, distributes it across the width of the headbox and sprays the suspension onto a wire screen belt of a following forming section of the paper machine. The headbox is intended to set the consistency, proportions of contents components, and fiber orientation transverse profile of the pulp stock suspension, so that the basis weight transverse profile, i.e., the cross machine profile and the fiber orientation transverse profile of the paper web produced from the suspension that exits the headbox corresponds to the desired requirements over the entire width of the web. Usually, these features are to be held constant over the width of the web. These parameters may be set at the headbox, at the latest before the exit of the suspension from the headbox.
In a dilution headbox, which is the type of headbox in this invention, the consistency of the pulp suspension is adjusted locally in sections across the headbox in order to produce a desired basis weight cross profile in the paper web produced. The narrower that the influenced width of an affected section of the paper web can be made, the better or more close to the ideal will be the cross profile of the basis weight.
When there is a goal of producing a desired fiber orientation cross profile in the web produced, then the flow rate, rather than the consistency, is adjusted locally in sections across the headbox. In the headbox, the narrower that the influenced width of an affected section of the paper web can be made, the better or more close to the ideal will be the fiber orientation cross profile of the web will be. For both the basis weight cross profile and the fiber orientation cross profile of the web, the narrower is each section of the headbox across its width, the closer to ideal will be the respective cross profiles of the web.
For this purpose, a modern headbox is equipped with sectional liquid supplies, i.e. respective controlled supplies of the suspension at the different sections across the width of the entrance to the headbox. This makes it possible, for example, to supply wire water, called "white water", i.e. water recovered from the dewatering of the suspension at the forming section, and to supply that water, individual section by section into the headbox. This influences the density of the stock or suspension flow in each section. As a result, this influences the web thickness in each section of the web across the web and also the basis weight of the web in each section across the paper web and/or, by means of section by section supply of stock suspension, this generates transverse flows in the headbox which influences the fiber orientation transverse profile of the web produced. To achieve as narrow width, influenced section across the width of the paper web as practicable, in relationship to the quantity of individual sections and the widths of the individual sections across the headbox at which consistency and/or flow rate is adjusted, it is desirable to avoid or at least minimize mixing of the sectional flows of adjacent sections over the headbox.
A modern headbox known, for example, from German patent specification DE 43 20 243 C2 comprises a machine width stock supply including a transverse distributor that delivers suspension across the inlet to the headbox, one or two turbulence inserts inside the headbox with, if appropriate, an intermediate chamber located between the inserts, and an outlet nozzle after the inserts. Furthermore, a multiplicity of individually adjustable supply lines of various liquids are provided in the region of and typically upstream of the turbulence inserts. The adjustable supply lines can supply wire water, or stock suspension or other additives to the main flow of the headbox, section by section over the width or over both the width and the height of the headbox.
This configuration of a headbox is disadvantageous, particularly for embodiments in which the section by section supply enters a channel in the headbox, because the channel is not limited laterally (FIGS. 6, 9, 10, 12 of DE 43 20 243 C2) and there are no guiding elements over the width of the chamber. The effective width of each sectional liquid supply is not limited laterally and, in addition, it is also dependent on the quantity of the liquid supplied. First, second, etc., sectional flows over the width mix laterally. As a result, the effective width of each sectional flow is too broad. A similar effect is seen with the headbox in U.S. Pat. No. 5,196,091. Hence, the basis weight cross profile of the web produced is not as well controlled as possible over narrow width sections across the web.